Study On The Nitrogen Removal And N₂O Emission Characteristics Duirng Shortcut Simultaneous Nitrification And Denitrification Process

With the rapid economic development, the nitrogen pollution in water environment is becoming increasingly serious. The applications of conventional biological nitrogen removal processes are limited due to its complex operation and large space requirement. Shortcut simultaneous nitrification and denitrification (shortcut SND) process is a novel and potential biological nitrogen removal process, which have advantages of saving extra carbon source, and reducing both aeration and production of sludge. However, shortcut SND process might release large amount of N2O, which is a kind of greenhouse gas, as the set-up of shortcut SND process require high standard environmental conditions. In this study, shortcut SND process was achieved in a designed air lift circulation bioreactors. The nitrogen removal and N2O emission characteristics of the shortcut SND process was investigated, corresponding results will act as a reference to the optimization of relevant processes.Primarily, based on the completed SND condition, the system could be switched to shortcut SND as the free ammonia (FA) concentration was increased in higher influent pH. The nitrogen removal and N2O emission characteristics of the shortcut SND process were investigated and those of the complete SND process were also observed as control sample. In shortcut SND process, the average total nitrogen removal and average SND efficiency was71.9%and80.9%, which was18.0and16.8percents higher than those in completed SND process, respectively. In addition, the total nitrogen removal rate in shortcut SND process was0.11mg·(L·min)-1,1.4times as much as that in complete SND process. The mean N2O conversion rate of the shortcut SND process could reach57.1%and the average N2O accumulated emission amount was approximately4times higher than that in the complete SND process. The results also indicated that the increase of NO2--N concentration in the reactor should be responsible for the remarkable enhancement of N2O emission.The effect of dissolve oxygen on the nitrogen removal and N2O emission during the shortcut SND process were investigated. The results showed that lower DO concentration could increase the nitrogen removal efficiency but slow down the nitrogen removal rate as DO concentration is at the range of0.5-2.0mg·L-1. The N2O conversion rate declined with the decrease of DO concentration. The N2O conversion rate dropped from71.2%to44.5%, when DO concentration decreased from2.0mg·L-1to0.5mg·L-1. The result also showed that heterotrophic denitrification was the main mechanism for N2O production in shortcut SND process. Base on the results of nitrogen removal efficiency, nitrogen removal rate and N2O conversion rate, the optimized DO concentration range for shortcut SND process should be0.8～1.2mg·L-1.The effect of different carbon sources and carbon to nitrogen (C/N) ratios on the nitrogen removal and N2O emission characteristics of the shortcut SND process were also studied. The results showed that carbon sources had great impact on the nitrogen removal and N2O emission characteristics. Among glucose, maltose and starch, the reactor achieved the maximum nitrogen removal efficiency and the minimum N2O conversion rate when glucose was used as carbon source, while the minimum nitrogen removal efficiency and the maximum N2O conversion rate in the system with additional carbon source of starch. Improvement in both total nitrogen removal and SND efficiency and reduction of N2O conversion rate could be achieved by increasing C/N ratio. However, the ammonia nitrogen removal rate and total nitrogen removal rate began to fall when the C/N ratio ascended to9. Considering the stability of reactor operation and carbon source minimization, the optimized C/N ratio should be7, with the average ammonia nitrogen removal rate, total nitrogen removal rate, SND efficiency and N2O conversion rate were100%,79.1%,88.2%and15.7%, respectively.These results suggest that shortcut SND process was a high efficiency biological nitrogen removal process with a considerable amount ofN2O emission. However, a sound shortcut SND process with high nitrogen removal and limited N2O emission can be achieved through the optimization of operation conditions, i.e., additional carbon sources and dissolved oxygen.